Abstract
In November 2014 Uranus was observed with the Wide Field Camera 3 (WFC3) instrument of the Hubble Space Telescope as part of the Hubble 2020: Outer Planet Atmospheres Legacy program, OPAL. OPAL annually maps Jupiter, Uranus and Neptune (and will also map Saturn from 2018) in several visible/near-infrared wavelength filters. The Uranus 2014 OPAL observations were made on the 8/9th November at a time when a huge cloud complex, first observed by de Pater et al. (2015) and subsequently tracked by professional and amateur astronomers (Sayanagi et al., 2016), was present at 30–40°N. We imaged the entire visible atmosphere, including the storm system, in seven filters spanning 467–924 nm, capturing variations in the coloration of Uranus’ clouds and also vertical distribution due to wavelength dependent changes in Rayleigh scattering and methane absorption optical depth. Here we analyse these new HST observations with the NEMESIS radiative-transfer and retrieval code in multiple-scattering mode to determine the vertical cloud structure in and around the storm cloud system.The same storm system was also observed in the H-band (1.4–1.8 µm) with the SINFONI Integral Field Unit Spectrometer on the Very Large Telescope (VLT) on 31st October and 11th November, reported by Irwin et al. (2016, 10.1016/j.icarus.2015.09.010). To constrain better the cloud particle sizes and scattering properties over a wide wavelength range we also conducted a limb-darkening analysis of the background cloud structure in the 30–40°N latitude band by simultaneously fitting: a) these HST/OPAL observations at a range of zenith angles; b) the VLT/SINFONI observations at a range of zenith angles; and c) IRTF/SpeX observations of this latitude band made in 2009 at a single zenith angle of 23°, spanning the wavelength range 0.8–1.8 µm (Irwin et al., 2015, 10.1016/j.icarus.2014.12.020).We find that the HST observations, and the combined HST/VLT/IRTF observations at all locations are well modelled with a three-component cloud comprised of: 1) a vertically thin, but optically thick ‘deep’ tropospheric cloud at a pressure of ∼ 2 bars; 2) a methane-ice cloud based at the methane-condensation level of 1.23 bar, with variable vertical extent; and 3) a vertically extended tropospheric haze, also based at the methane-condensation level of ∼ 1.23 bar. We find that modelling both haze and tropospheric cloud with particles having an effective radius of ∼ 0.1 µm provides a good fit the observations, although for the tropospheric cloud, particles with an effective radius as large as 1.0 µm provide a similarly good fit. We find that the particles in both the tropospheric cloud and haze are more scattering at short wavelengths, giving them a blue colour, but are more absorbing at longer wavelengths, especially for the tropospheric haze. We find that the spectra of the storm clouds are well modelled by localised thickening and vertical extension of the methane-ice cloud. For the particles in the storm clouds, which we assume to be composed of methane ice particles, we find that their mean radii must lie somewhere in the range 0.1−1.0μm. We find that the high clouds have low integrated opacity, and that “streamers” reminiscent of convective thunderstorm anvils are confined to levels deeper than 1 bar. These results argue against vigorous moist convective origins for the cloud features.
Highlights
Long-term observations of the outer planets are critical to understanding the atmospheric dynamics and evolution of gas giant planets (Visions and Voyages pp.78-86)
Since to describe fully the vertical distribution of cloud opacity and determine the cloud particles sizes and scattering properties requires many more than seven pieces of information it is clear that we must first constrain the analysis of the Hubble Space Telescope (HST)/Wide Field Camera 3 (WFC3) observations by constructing a parameterised model of Uranus’ clouds that can be represented with only a few variables. We could do this for the HST/WFC3 observations alone, but limiting the analysis to a restricted wavelength range could lead to solutions that might be consistent with the HST/WFC3 data, but which are inconsistent with observations at other wavelengths
We applied our model to fitting the best resolved Very Large Telescope (VLT)/SINFONI observations recorded on 31st October 2014, reported by Irwin et al (2016), again fixing the tropospheric cloud and tropospheric haze opacities to the values determined from the limb-darkening study, and allowing only the optical depth and fractional scale height of an additional methane cloud to vary
Summary
Long-term observations of the outer planets are critical to understanding the atmospheric dynamics and evolution of gas giant planets (Visions and Voyages pp.). The bright cloud seen by amateurs was not the very bright ‘Br’ feature seen by de Pater et al (2015), but instead seemed to have evolved from a fainter ‘Feature 2’, seen at 33◦N These detections prompted a number of ground-based Director’s Discretionary Time (DDT) observations at the world’s leading observatories, including observations in the H-band (1.4–1.8 μm) made with the SINFONI integral field unit spectrograph at the European Southern Observatory (ESO) Very Large Telescope (VLT) by Irwin et al (2016). Using these data we attempt to find aerosol vertical distributions that are simultaneously consistent with both these observations and earlier IRTF/SpeX Uranus observations, made in 2009 (Tice et al 2013)
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